1. Field of the Invention
The present invention relates to a piezoelectric electroacoustic transducer such as a piezoelectric receiver and a piezoelectric sounder.
2. Description of the Related Art
Conventional piezoelectric electroacoustic transducers are commonly used in electronic devices, household electric appliances, mobile phones, etc., as piezoelectric sounders for producing alarm sounds and operation sounds or as piezoelectric receivers. Piezoelectric electroacoustic transducers are generally constructed by adhering a circular piezoelectric device on a circular metal plate so as to define a unimorph-type diaphragm, installing the metal plate in a circular case such that the metal plate is supported by silicone rubber at the periphery thereof, and closing an opening of the case with a cover.
However, when a circular diaphragm is used, there are problems in that production efficiency and efficiency in electroacoustic conversion are low and it is difficult to reduce the size of the device.
Accordingly, Japanese Unexamined Patent Application Publication No. 2000-310990 discloses a surface-mountable piezoelectric electroacoustic transducer in which a diaphragm having a rectangular shape is used so that the production efficiency and the efficiency in electroacoustic conversion can be increased and the size of the device can be reduced. This piezoelectric electroacoustic transducer includes a piezoelectric diaphragm having a rectangular shape, an insulating case having supporters for supporting the diaphragm between two opposing sides of the case and terminals which are attached to the supporters and provide connections to the external environment, and a cover plate in which a sound-emitting hole is formed. The diaphragm is installed inside the case, and two opposing side surfaces of the diaphragm are fixed to the supporters with an elastic sealing material. In addition, clearances between the remaining two sides of the diaphragm and the case are sealed with the elastic sealing material. First and second conductive members are electrically connected to the diaphragm with a conductive adhesive, and the cover plate is adhered on the case so as to close an opening of the case disposed at one side thereof.
Although the above-described piezoelectric electroacoustic transducer uses a unimorph-type piezoelectric diaphragm, a piezoelectric electroacoustic transducer using a piezoelectric diaphragm including a piezoelectric ceramic having a laminated structure is also known in the art (Japanese Unexamined Patent Application Publication No. 2001-95094).
According to the known art, the diaphragm is fixed and connected to the terminals, which are fixed to the case, by a conductive adhesive at two sides thereof. Therefore, there is a problem in that the diaphragm is strongly restrained by the case, and if, for example, thermal deformation of the case occurs, it directly affects the diaphragm. When the case is mounted on a printed board by reflow-soldering, thermal expansion of the case occurs. In such a case, characteristics of the diaphragm change due to the difference between the coefficient of thermal expansion of the case and that of the diaphragm. In addition, when a stress is applied to the case, the stress is directly transferred to the diaphragm. Accordingly, there is a risk in that characteristics of the diaphragm will change or the diaphragm will break.
In order to overcome the problems described above, preferred embodiments of the present invention provide a piezoelectric electroacoustic transducer in which thermal deformation of the case and a stress applied to the case do not affect the diaphragm and in which the change in characteristics of the diaphragm is minimized.
According to a preferred embodiment of the present invention, a piezoelectric electroacoustic transducer includes a piezoelectric diaphragm having a substantially rectangular shape which vibrates in the thickness direction when an-alternating current is applied between electrodes of the piezoelectric diaphragm, a housing which stores the piezoelectric diaphragm and which includes supporting portions for supporting the piezoelectric diaphragm at at least two opposing sides of the piezoelectric diaphragm or at corners of the piezoelectric diaphragm, and a pair of terminals disposed and insert molded in the housing such that a first end of each terminal is inserted inside the housing at a position close to the supporting portions and a second end of each of the pair of terminals is disposed outside the housing. The first end of each of the pair of terminals includes a body portion fixed to the housing, wing portions which extend from both sides of the body portion toward the corners of the housing and which are not fixed to the housing, and stress-relieving portions disposed between the body portion and the wing portions such that the wing portions are able to move toward the inside of the housing. Each electrode of the piezoelectric diaphragm is connected to at least one of the wing portions of the terminals.
According to the present preferred embodiment of the present invention, one end of the terminal which is inserted inside the housing is provided with a body portion and wing portions which extend from both sides of the body portions. The body portion is fixed to the housing and the wing portions are able to move relative to the housing. In addition, stress-relieving portions are disposed between the body portion and the wing portions so that the wing portions are able to move toward the inside of the housing. At least one of the wing portions is connected to one of the electrodes of the piezoelectric diaphragm preferably via a conductive adhesive. When a temperature change occurs or an external force is applied, a stress generated due to the temperature change or the external force is transferred toward the piezoelectric diaphragm. However, the wing portions of the terminals are bent toward the inside of the housing so as to absorb the stress, so that the stress is prevented from being transferred to the piezoelectric diaphragm. Therefore, the piezoelectric diaphragm is of course prevented from breaking and electrical characteristics of the piezoelectric diaphragm do not change. Accordingly, the stability of the electrical characteristics of the diaphragm is reliably ensured.
Preferably, end surfaces of the wing portions of each terminal are inclined such that the distance between the end surfaces is increased toward the inside of the housing.
When the terminals are formed by insert molding, end surfaces of the wing portions are surrounded by a resin. Accordingly, when the end surfaces of the wing portions are substantially perpendicular to the inside surface of the housing, there is a risk in that the wing portions will be impeded from moving toward the inside by the resin surrounding the end portions of the wing portions. Accordingly, the end surfaces of the wing portions are inclined so that the wing portions will not be impeded from moving toward the inside of the housing.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.